The principal objective of an air-conditioning system for human occupancy in buildings is to provide a controlled comfortable and healthy indoor environment. Other objectives of an air-conditioning system can be to provide for the special requirements of storage, processes or equipment operation within a building.
The fundamental variables for an air-conditioning system to control in a defined indoor zone are air temperature, air humidity, air movement and air quality.
The work of scientists has shown that human comfort (also called thermal acceptability) is achieved, when the values of air dry bulb temperature, mean radiant temperature, humidity and air movement conform to a defined relationship called a "comfort equation".
This equaltion also incorporates the additional variables of human activity and clothing level.
Comfort indoors or other required conditions are achieved by the invention described herein by controlling air dry bulb temperature, humidity and air movement in the occupied space.
An advantageous characteristic of the invention when used for cooling particularly in comfort applications is the ability to provide a larger supply air quantity at higher dry bulb temperature while maintaining dehumidification but without the energy penalty associated with traditional heat pump systems attempting the same process and thereby reducing their dehumidifying capacity. The advantages in this characteristic of the invention are threefold and interrelated. They are:
(1) By increasing supply air quantity (over that traditionally used) and thereby permitting higher air dry bulb temperature (than traditionally used) while being able to provide comfort conditions in accordance with the aforementioned "Comfort Equation". PA0 In ASHRAE Standard 55-1981, the relationship between air movement, air dry bulb temperature and comfort is shown to be such that an increase in air movement of 0.275 m/s would allow an increase in air dry bulb temperature of 1.degree. K. for the same comfort condition. The subject invention readily allows an increase of 2.degree. K. for the same comfort conditions, (i.e. an increase in air movement of 0.55 m/s). PA0 (2) By increasing supply air dry bulb temperature there is a reduction in the heat transmission through the building envelope as a result of the smaller temperature difference between outside and inside the building. The quantity of heat transmitted is directly related to this temperature difference thus the lower the heat quantity the less energy is required in the cooling system to extract it from the building. PA0 (3) By increasing the supply air temperature (above that traditionally used) damage to internal building surfaces which frequently occurs in tropical climates can be avoided. It is often unavoidable that the supply air, as it enters the occupied space through the distribution system, impinges directly on internal wall or ceiling surfaces. If this impingement occurs prior to mixing with or entraining inside air to thereby, in turn, raise the mixed temperature above dew point temperature, then the building surface can become cold enough for moisture to condense on it and cause damage by staining and increase in mass leading to structural failure. PA0 (a) The high energy penalty incurred in providing sufficient ventilation air to ensure an acceptable healthy indoor environment for both heating and cooling. PA0 (b) The lack of control of humidity that is typical of traditional air conditioning systems unless reheating of the supply air after cooling is sued. Supply air reheat systems, as a first step, add extra energy to cool the supply air to a lower than necessary dry bulb temperature in order to reduce the humidity and then, as a second step, add energy to heat the supply air to the required dry bulb temperature. Both steps are wasteful of energy. Reheat systems using heat rejected from the heat pump are costly to install and difficult to control. The control of the lower limit of humidity requires extra humidifying equipment to add moisture vapour to the supply air. Such equipment requires the addition of extra energy, usually direct electrical, and is thus energy inefficient. Reheating and humidifying systems are costly both to install and operate. PA0 (c) The high condensing temperatures in air cooled heat pumps during peak cooling demand. These cause excessive energy consumption and performance fall off. Cooling performance reduces as ambient air temperature increases and since ambient air is at highest temperature when cooling need is greatest for most applications particularly in hot climates, energy consumption is highest under these conditions. PA0 (d) The failure of air source heat pumps to heat efficiently in ambient air temperatures of about 6.degree. or less is due to ice forming on the heat exchanger coils. This causes a need to provide high energy consuming electric heating elements or alternative heating means to make up for the heat pump performance reduction. Performance needs to be greatest when ambient temperatures are lowest in most applications. PA0 (e) The need in water cooled heat pumps during cooling to require extra equipment such as cooling towers to provide a continuous supply of cool water at suitable temperature for condensing. This increases the cost of the installation. PA0 (f) The need in water source heat pumps used for heating supply air, to require extra equipment such as storage tanks and solar heat exchangers. Alternatively a stream or pond that is always above about 6.degree. is required from which heat can be extracted without it freezing. PA0 (a) Cooling of supply air without dehumidification or humidification. PA0 (b) Cooling of supply air with dehumidification. PA0 (c) Cooling of supply air with humidification. PA0 (d) Heating of supply air without humidification. PA0 (e) Heating of supply air with humidification.
An associated problem occurs if the supply air temperature has, at the same time, such a low dew point temperature that it reduces the vapour pressure of the moisture inside the building surface and thus draws moisture through the surface from the higher vapour pressure outside. Suitably positioned moisture impervious materials can prevent these problems of building damage but are costly to install.
These damaging conditions occur with traditional heat pump systems that are unable to prevent an unnecessarily low level of humidity in the supply air discharged.
The subject invention, by allowing higher temperature in the supply air and by preventing an unnecessarily low humidity in the supply air, positively prevents the problems described above that occur particularly in tropical climates. It will be realised also that to have too low a humidity can cause other occupational dangers such as the generation of static electricity and an increase in respiratory disease.
Indoor air quality for providing a healthful environment has recently received considerable scientific attention since numerous types of illness have been attributed to indoor air contamination.
Recent studies have not only identified and quantified new indoor polluting materials and their effects but also have more closely examined traditional pollutants such as from cooking, smoking and naturally occurring radio-active radon gas as well as disease causing organisms.
New pollutants are principally formaldehyde, chlorinated organic chemicals, asbestos fibre and combustion products. Some of these indoor generated contaminants can be toxic and carcinogenic.
On the other hand, outdoor air in most environments is low in all contaminants since much community effort and legislation has been aimed at maintaining that condition. Ventilation by outside air is very effective in removing indoor generated pollution and to provide oxygen make-up.
Generally, outside air needs only simple filtration to achieve a suitable quality for ventilation purposes. The removal of indoor generated pollutants other than by dilution or replacement is generally a difficult and costly chemical process and is rarely justified unless the energy cost of heating and cooling outside sourced ventilation air is excessive.
The invention described herein greatly reduces the energy required for heating and cooling ventilation air and obviates the need to consider indoor air treatment except for applications in unusually polluted environments.